Valves have been in use heretofore in vehicle rear wheel brake circuits for reducing the hydraulic braking pressure applied to the rear wheel brake cylinders such that it is less than that applied to the front wheel brakes, with the concept of preventing the rear wheels from being locked earlier than the front wheels to prevent skidding of the vehicle rear end. However, in reducing valves of the conventional type wherein the prevention of rear end skidding is limited to vehicles calculated to have a prescribed or preset amount of load, a drawback appears in that sufficient prevention of skidding cannot be expected if the vehicle load is lighter than said prescribed amount of load, and, on the other hand, the braking force will be found to be insufficient if the load is heavier than the prescribed amount. A load-responsive reducing valve is used in order to eliminate this drawback.
These load-responsive reducing valves may roughly be classified into two types, i.e., the load detection type and the deceleration detection type.
However, in the case of said load detection type wherein the sinking of the vehicle loading platform by the load or the hydraulic pressure generated by the load is directly transmitted mechanically to the reducing valve to modulate the performance of the reducing valve the line or direction of the applied force on the reducing valve is liable to change or become inclined and the reduction starting pressure is therefore liable to be changed irradically by the vibration of the vehicle due to the unevenness of the road surface or irregular sinking of the vehicle under braking. Furthermore, there has also been a drawback in that troublesome adjustment is required when equipping the vehicle with the device.
In the case of said deceleration detection type of pressure reduction valve, wherein an inertia-operated ball which starts to actuate the reducing valve when the deceleration of the vehicle reaches a predetermined value, and the response to the load is affected indirectly by means of a rise in hydraulic braking pressure at the time of increased load on the vehicle, it is not suitable for vehicles that are not sensitive to the difference in deceleration caused by different applied loads on the vehicle.
The present invention relates to a load-responsive reducing valve which has a means to correct the shortcomings of the reducing valves of said load detection type.
As for the aforementioned prior art reducing valves of the load detection type, U.S. Pat. No. 3,169,800 and U.S. Pat. No. 3,362,758 disclose known examples. As for the deceleration detection type, U.S. Pat. No. 3,317,251 discloses a known example.
A reducing valve wherein the pressure reducing plunger of the reducing valve is controlled by the hydraulic pressure of a control chamber is disclosed in Japanese Patent Application Laid-Open No. 49-37066 whose inventors include the inventor of the present invention.